Major points: donations are used for projects working on things like web literacy, rather than on large engineering projects like Firefox. The more Mozilla get from donations, the less dependent on Google/Yahoo they have to be and thus they can be more independent - even fairly small absolute amounts will 'push the needle' here.

Minor point: I strongly contest that Mozilla is a wasteful company - salaries are low by large-tech-firm standards and spending (on travel, equipment, etc.) is fairly frugal. I have never heard of employees having almost work to do - it is nearly always the opposite. Travel is nearly always for coordinated team work since many employees and volunteers work remotely. For a (relatively) small budget, Mozilla produce an awful lot of results - the entire company is run on vastly less than the marketing budget (!) for Chrome, for example (sorry, can't find a source though).